Method and apparatus for welding polymer fabrics

ABSTRACT

The apparatus is adapted to weld ultrasonically two or more layers of fabric  14.  The fabrics  14  may be a woven or non-woven fabric of nylon, polyethylene or other polymeric material. It comprises two opposed ultrasonically vibratable horns ( 1, 1   a ) so spaced one from another that the layers of fabric ( 3 ) may be passed between them. At least one, optionally both, of the two horns ( 1, 1   a ) is vibratable in axial mode.

[0001] The present invention relates to a method and apparatus forwelding polymer fabrics using ultrasound. More particularly, but notexclusively, it relates to a method and apparatus for welding polymerfabrics or the like comprising a pair of ultrasonically vibrated hornsacting on layers of fabrics to be welded.

[0002] It is well known that certain polymers absorb ultrasound readilyand by virtue of that property, offer scope for fusion bonding and otherjoining techniques. These phenomena have established a generalapplication in the welding of engineering plastics, covering a widerange of techniques and encompassing products in many different markets.Of equal interest but less widely practised is the application ofultrasound to welding of woven or non-woven polymer fabrics.Considerable effort has been expended in developing ultrasoundprocessing equipment for the manufacture of high strength non-woven orreinforced fabrics and specialised elasticated products.

[0003] One method of undertaking ultrasonic welding is disclosed by thepresent applicant in an earlier patent number GB 2299538 B, but othersimilar procedures are also known from U.S. Pat. No. 835,068, WO94/11189, U.S. Pat. No. 249,416, U.S. Pat. No. 4,400,227, GB 8805949,U.S. Pat. No. 4,305,988, FR 2255023 and WO 95/09593.

[0004] However, not all the methods disclosed are entirely satisfactoryand it is therefore an object of the present invention to provide amethod and apparatus for welding polymer fabrics using ultrasound, whichmethod is both effective, quick and is able to deal with differentmaterials and different thicknesses of material.

[0005] Preferred polymer fabrics to be welded by the method andapparatus of this invention are those containing nylon and polyethylene,although other materials may also be welded.

[0006] According to a first aspect of the present invention, there isprovided an apparatus for welding ultrasonically two or more layers offabric comprising two opposed ultrasonically vibratable horns so spacedone from another that the layers of fabric may be passed between them.

[0007] Preferably at least one, optionally each, of the two horns isvibratable in axial mode.

[0008] Where both horns are vibratable in axial mode, the relative phasebetween the vibrations of the horns may be varied, whereby for constantamplitude vibrations the energy imparted may vary between a minimum atphase difference Ø=0° and a maximum at phase difference Ø=180°.

[0009] Alternatively the frequencies at which the horns are adapted tovibrate may be so varied that one is vibratable at a frequency plus orminus between 1 and 20%, preferably in the region of 10%, of thefrequency of the other.

[0010] In another embodiment, an axially vibratable horn may have anangled end face, and a second axially vibratable horn may vibrate in adirection substantially perpendicular to said end face.

[0011] The welding apparatus described above may further include asource of air directable generally toward a zone in which welding takesplace.

[0012] The source of air may be a passageway extending substantiallylongitudinally of at least one of the welding horns to exitsubstantially centrally of the welding zone.

[0013] Alternatively the source of air or other cooling fluid may be apassageway longitudinal of one or both welding horns, the or each saidpassageway difurcating to permit exit of air through a plurality ofannularly spaced outlets.

[0014] In another embodiment where one horn is vibratable in an axialmode, an opposed co-operating horn is vibratable in a torsional mode.

[0015] According to a second aspect of the present invention, there isprovided a method of welding ultrasonically two or more layers of fabriccomprising the steps of providing two opposed ultrasonically vibratablehorns spaced one from another, and passing the layers of fabric betweenthem whilst vibrating ultrasonically the horns.

[0016] Preferably at least one, optionally each, of the two horns isvibrated in axial mode.

[0017] Where both horns are vibrated in axial mode, the horns may bevibrated with a relative phase difference between them, said phase shiftbeing variable between Ø=0° for minimum energy and Ø=180° for maximumenergy.

[0018] Alternatively the horns may be so vibrated that one vibrates at afrequency plus or minus between 1 and 20%, preferably in the region of10%, of the frequency of the other.

[0019] In another embodiment, one horn is vibrated in an axial modewhilst an opposed co-operating horn is vibrated in a torsional mode.

[0020] The method described above further includes the step of directingair or other cooling fluid toward a zone in which welding takes place.

[0021] Embodiments of the present invention will now be moreparticularly described by way of example and with reference to theaccompanying drawings, in which:

[0022]FIG. 1 shows a pair of axial mode vibratable horns inlongitudinally opposed relationship and, graphically, the effect ofphase control thereon;

[0023]FIG. 2 shows welding by one axial mode horn opposed by a torsionalmode horn;

[0024]FIG. 3 shows welding by an axial mode vibratable horn having anangled face and a second horn vibratable in a direction perpendicular tosaid angled face;

[0025]FIG. 3A is an end view of the horn of FIG. 3;

[0026]FIG. 4 shows an opposed pair of longitudinally vibratable horns,each having a central passage for introduction of cooling air;

[0027]FIG. 5 shows an opposed pair of vibratable horns, each having acentral passage for introduction of cooling air which passage splitsinto a plurality of annularly spaced outlets; and

[0028]FIG. 6 is a graph of breaking strength against weld speed, undervarious welding conditions.

[0029] Since the mechanism of fabric welding is dependent on generatingheat in the workpiece, the oscillatory process should be designed tooptimise this effect. The ideal process would result in adjacent fibresbeing fused locally rather than in bulk. This would form a union whichwas flexible and retained the “feel” of fabric, rather than giving asolid fused mass of amorphous material. Control of the welding processis conventionally effected by varying the ultrasonic power cycle and theforce applied between the welding tool and supporting anvil placedbeneath the workpiece. With fabrics having relatively fine fibres itwould be a great advantage to have additional parameters allowing moredelicate control of the fusion process.

[0030] One technique which would encourage this objective requires botha horn and an anvil horn to be vibrated but at different frequencies.One, say the anvil frequency, is deliberately moved to be approximately300 Hz higher or lower than the frequency of the other. As a result,phase shifted opposed vibrations of ultrasonic energy would be appliedat a frequency represented by the difference between the two systems,(namely one at 28 kHz, and the other at say 28.3 kHz or 27.7 kHz). Thisbeating effect would greatly reduce the energy for a given amplitude of,say c. 80 μm, to about 100 Watts for each transducer, and permit handcontrolled welding of small delicate materials which necessarilyinvolves slower relative translational movement of fabric and tools.

[0031] Referring now to FIG. 1, this illustrates the effect on energyabsorption of phase shift between two longitudinally opposed horns 1 and1 a, each in axial vibrational mode. By merely controlling the phaseangle the weld energy can be varied between a minimum at Ø=0° and amaximum at Ø=180°, whilst maintaining stable horn resonance.

[0032] The application of vibrations simultaneously via both an anvilhorn and a welding horn with phase shift between the two energisedsystems has the effect of varying the energy absorption from a minimum,probably zero, at 0° phase angle to a maximum at 180°.

[0033] This principle utilises two systems each operating in thelongitudinal mode. However, if the horn is excited longitudinally andthe anvil is excited in a transverse or orbital mode the resultant weldcharacteristic will be quite different. Relative oscillatory movementbetween adjacent fibres will produce very local heating and by definingcarefully the contact pressure and energy input, local fusion can beproduced in a controlled way.

[0034] This effect can be seen in FIG. 2 which shows an axial mode horn1 with counterposed torsional mode horn 2. The fabric layers 3 aretransported through a gap between the horns 1 and 2 in the directionindicated by arrow 4.

[0035] The effect of the torsional mode excitation at annular surface 5rotates the fibres of the fabric whilst they are simultaneously beingcompressed by the axial displacement of horn 1 under direct pressure P.This oscillatory motion, if sufficiently energetic, creates frictionheating between fibres, sufficient to induce fusing without the need tocompress fully the fabric layers.

[0036]FIG. 3 illustrates two opposed horns with the fabric layerspassing through them at an angle. In this case, one of the horns 7 isvibratable in an axial mode and has an angled end face 8 whilst theother horn 6 is vibratable in an axial mode perpendicular to the planeof the end face 8 of the first horn 7. Given the directions ofvibrations of the horns, the offset angle between them causes localisedheating and some rearrangement of the fibres of the fabrics passingbetween them, thereby welding the fabrics together.

[0037] There is shown in FIGS. 4 and 5 an air cooled welding system inwhich air is introduced through vibratable horns 9. In FIG. 4, the airpasses along tube 10 arranged axially to the horn or horns 9 to bedischarged centrally of the welding zone α. In FIG. 5, the passages 10each split into a plurality of passages, which exit through acorresponding plurality of holes 15 arranged annularly to surround thecentre of the welding zone α.

[0038] In both FIGS. 4 and 5, the passage of the materials to be weldedis shown as 16. In both cases, alternative forms of introduction ofcooling fluid are possible.

[0039]FIG. 6 is a graph of breaking strength versus weld speed fordifferent horn characteristics. High gain horns with air cooling producegood weld strength with low speed sensitivity. The results without airsuggest that careful balance of the energy input per unit volume ofmaterial (a function of speed) with the cooling effect of injected air,could lead to optimised weld quality. The objective of high speedwelding with consistent high strength could be achieved with arelatively simple mechanical system.

1. An apparatus for welding ultrasonically two or more layers of fabriccomprising two opposed ultrasonically vibratable horns so spaced onefrom another that the layers of fabric may be passed between them.
 2. Anapparatus as claimed in claim 1, wherein at least one of the two hornsis vibratable in axial mode.
 3. An apparatus as claimed in claim 2,wherein both horns are vibratable in axial mode.
 4. An apparatus asclaimed in claim 3, wherein the relative phase between the vibrations ofthe horns is so variable that, for constant amplitude vibrations, theenergy to be imparted varies between a minimum at phase difference Ø=0°and a maximum at phase difference Ø=180°.
 5. An apparatus as claimed inclaim 3, wherein the frequencies at which the horns are adapted tovibrate are so variable that one is vibratable at a frequency plus orminus between 1 and 20% of the frequency of the other.
 6. An apparatusas claimed in claim 5, wherein one horn is adapted to vibrate at afrequency plus or minus 10% of the frequency of the other.
 7. Anapparatus as claimed in any one of claims 2 to 6, wherein one axiallyvibratable horn has an end face angled with respect to the vibrationalaxis, and the second axially vibratable horn is vibratable in adirection substantially perpendicular to said end face.
 8. An apparatusas claimed in claim 2, wherein one horn is vibratable in an axial modeand an opposed co-operating horn is vibratable in a torsional mode. 9.An apparatus as claimed in any one of the preceding claims, furthercomprising a source of air or other cooling fluid directed generallytoward a zone in which welding takes place.
 10. An apparatus as claimedin claim 9, wherein the source of air or other cooling fluid comprises apassageway extending substantially longitudinally of at least one of thewelding horns to exit centrally of the welding zone.
 11. An apparatus asclaimed in claim 9, wherein the source of air or other cooling fluidcomprises a passageway extending longitudinally of at least one of thewelding horns, at least one of the or each said passageway difurcatingto permit exit of air or cooling fluid through a plurality of annularlyspaced outlets.
 12. A method of welding ultrasonically two or morelayers of fabric comprising the steps of providing two opposedultrasonically vibratable horns spaced one from another, and passing thelayers of fabric between them whilst vibrating ultrasonically the horns.13. A method as claimed in claim 12, wherein at least one of the twohorns is vibrated in axial mode.
 14. A method as claimed in claim 13,wherein both of the horns are vibrated in axial mode.
 15. A method asclaimed in claim 14, wherein the horns are vibrated with a relativephase difference between them, said phase shift being variable betweenØ=0° for minimum energy and Ø=180° for maximum energy.
 16. A method asclaimed in claim 14 wherein the horns are so vibrated that one vibratesat a frequency plus or minus between 1 and 20% of the frequency of theother.
 17. A method as claimed in claim 16, wherein the horns are sovibrated that one vibrates at a frequency plus or minus 10% of thefrequency of the other.
 18. A method as claimed in claim 13, wherein onehorn is vibrated in an axial mode whilst an opposed co-operating horn isvibrated in a torsional mode.
 19. A method as claimed in any one ofclaims 12 to 18, further comprising the step of directing air or othercooling fluid toward a zone in which welding takes place.